The objectives of this project are to characterize age-related differences in the in vitro responses of murine lymphohemopoietic cells to mitogens, circannual rhythmicity in their functional responses to mitogens and their differential susceptibility to freezing injury. Because there are optimal cooling velocities for the cryopreservation of each cell type, T- and B-lymphocytes from young animals can be recovered with slight impairment of function using an optimal cooling rate. Changes in cellular structure and/or function with age then may be distinguishable with selective freezing injury to one or both subpopulations. Structures considered the probable sites of freezing damage are the plasma membrane and cellular membrane systems.